1. Field of the Invention
The present invention relates to an antireflection film, a polarizing plate and an image display device.
2. Description of the Related Art
In general, an antireflection film is disposed on the outermost surface of image display devices such as cathode-ray tube display devices (CRT), plasma display panels (PDP), electroluminescent display devices (ELD) and liquid-crystal display devices (LCD). This is for preventing contrast reduction or image reflection owing to external light reflection on the displays, generally having a function of refractivity reduction owing to the principle of optical interference.
The antireflection film of the type is generally produced by forming, on a transparent support, a film of a low-refractivity layer having a suitable thickness and having a refractive index lower than that of the transparent support as an outermost layer of the film. For realizing its low refractivity, the material for the low-refractivity layer is desired to have a refractive index as low as possible. Since the antireflection film is disposed on the outermost surface of displays, it is desired to have high scratch resistance. In case where the support is a plastic film having a thickness of tens of μm to a few mm, a hard coat layer for compensating the poor indentation elasticity of the support is indispensable in addition to the strength of the film itself of the low-refractivity layer and the adhesiveness thereof to the underlying layer, for realizing high scratch resistance of the film. In particular, high scratch resistance has come to be required in recent applications such as TV panels and monitors, and the improvement of the indentation elasticity by a hard coat layer in addition to the strength of the film of the outermost layer is a significant theme in the art.
In addition, the recent tendency in the art is toward large-size high-definition TV panels and monitors, and it has become indispensable to make the hard coat layer have a function of light diffusion for retarding the display characteristic unevenness intrinsic to display devices. Two methods of making the layer have a function of light diffusion are known; one is an antiglare hard coat layer having both surface scatterability and internal scatterability (hereinafter this is referred to as “antiglare layer”), and the other is a diffusive hard coat layer having mainly internal scatterability (this is hereinafter referred to as “diffusive layer”). In both of the two, the mainstream is that resin particles or inorganic particles having a suitable refractive index and a suitable particle size are dispersed in the hard coat layer for attaining the light scatterability of the layer, as in JP-A 2007-264113 and JP-A 2005-187770.
As described in JP-A 2007-264113 and JP-A 2005-187770, the size of particles is an important factor for controlling the internal scatterability; but use of inorganic particles is problematic in that the precipitatability thereof is poor since the specific gravity thereof is high and that the film may be brittle since the hardness of the particles is high relative to that of the film. On the other hand, use of resin particles is also problematic in that the internal scatterability could be hardly in the optimum condition (in point of the refractive index and the particle size) since the particles are, differing from inorganic particles, swellable with solvent. In that situation, in order to make the layer have a controlled internal scatterability, it is desirable to lower as much as possible the swellability of the particles in solvent, as in JP-A 2007-148398, from the viewpoint of optical planning and production of the film. In this case where the swellability of the particles in solvent is reduced, however, it has been found that the optical film having the optimum internal scatterability in the initial state at room temperature and ordinary humidity is significantly degraded in point of its durability in a high-temperature high-humidity environment.